Pressurized water nuclear reactors comprise, inside a reactor building, a vessel containing the reactor core, filled with pressurized water, as well as a primary circuit consisting of several loops in communication with the vessel. Each of the primary circuit loops comprises a steam generator in which pressurized water is cooled while heating and vaporizing the feed water. The steam generators are placed in compartments known as bunkers, arranged inside the reactor building.
The steam generators of each of the loops are connected to the vessel by pressurized water inlet and outlet pipes known as primary pipes. These pipes are connected to the water box of the steam generator situated in its lower part, by means of pipework fixed to the water box.
After a certain period of operation, the steam generators which contain a bundle of heat exchange tubes responsible for the separation between the primary water and the feed water have to be checked and, if certain tubes of the bundle show leakages, these tubes must be closed by means of a plug or must be lined, to prevent contamination of the feed water.
When the steam generator has operated for a long time, it may be found necessary to replace some or all of the tubes of the bundle.
However, this operation is complicated, with the result that the complete replacement of the steam generators of pressurized water nuclear reactors is now being envisaged.
This replacement operation requires a sectioning of the primary pipes in the vicinity of the pipework connecting the worn-out generator and welding the pipework of the new replacement steam generator to the primary pipes left in preparation.
In the case where a new steam generator is installed in a nuclear power station under construction, the steam generator is placed in position in the bunker on vertical support props, and its connection to the reactor vessel and to the primary pump of the corresponding loop is then effected by means of primary pipe sections which are assembled end to end. Shimming of the steam generator in the bunker is then ensured by placing supporting devices around the steam generator, allowance being made for the mounting clearances which are required when the steam generator operates at high temperature.
The primary pipe sections responsible for connecting the loop members may be easily adapted in order to allow for the relative position of the components.
On the other hand, in the case where a worn-out steam generator of a nuclear reactor which has already been in operation is completely replaced, the new replacement generator has to be connected to an existing primary loop in which the worn-out steam generator is situated.
In theory, it is possible to envisage modifying the geometry of this loop, by sectioning considerable lengths of the primary pipes connecting the steam generator. However, there is a risk that the total duration of the operations enabling the steam generator to be replaced may be very long, and this correspondingly lengthens the period during which the nuclear reactor is not available. Furthermore, the primary pipes which are sectioned have been contaminated by the radioactive primary fluid, with the result that a decontamination must be carried out before work is done on the pipe, in all the regions where a sectioning operation is to take place.
It has therefore appeared preferable to connect the new steam generator to the primary circuit after having performed the sectioning of a minimum length of the primary connection pipes of the existing circuit.
Wherever possible, only a single sectioning of each of the primary connecting pipes is performed, in the vicinity of the pipework of the worn-out steam generator.
After extraction of the worn-out steam generator from the bunker, the ends of the pipes left in preparation are machined in order to produce on these ends chamfers in positions enabling them to be placed to coincide with the corresponding chamfers machined on the pipework of the new steam generator.
Certain unavoidable differences exist, furthermore, between the new steam generator and the worn-out steam generator, in respect of their geometry.
These differences influence the feasibility and the ease of implementing the procedure for replacing the steam generator, insofar as they affect the parts of the steam generator to be coupled to the primary circuit and the bearing or positioning means of the steam generator which interact with the corresponding supporting and positioning means which remain in position in the bunker.
Until now there was no known procedure which made it possible to ensure the complete replacement of a worn-out steam generator of a pressurized water nuclear reactor, with an accurate determination of the sectioning to be carried out, of the position of the chamfers for welding and of the modifications to be made to the supporting and holding device.